Celebrities do things to make news. They fall in love and fall out of love in as much time as it takes me to decide which lipstick to buy from where. So what's the big deal about a "handful" of celebrities exiting Twitter? They may come back, they may really be busy, they may realize that tweeting is adding to the rumor mill and not really helping them.
A celebrity on Twitter generally tweets ONE-WAY! I for one find that to be so not about networking and SO about self-promotion. So whether they stay or go, who cares?
I must say that I can kinda relate to what Kanye West was quoted as saying back in May:
"In May, rapper Kanye West ranted against the service, writing in his "Caps Lock" glory, "I'M TOO BUSY ACTUALLY BUSY BEING CREATIVE MOST OF THE TIME AND IF I'M NOT AND I'M JUST LAYING ON A BEACH I WOULDN'T TELL THE WORLD. EVERYTHING THAT TWITTER OFFERS I NEED LESS OF.""
Bottom line is if you have time and feel the need or the desire, then you should tweet or facebook or be linkedin. If you don't, then don't!